Imagine, for a second, that you are in a casino in Las Vegas. You have been in town for a few days, spent too much and slept too little, and recently found out the hard way that you are not as good at poker as you had thought. Now would be a good time to find a wager where the odds, like a tipsy bartender, are tilted in your favor.

Here's a hint: find your way to the sports betting parlor. If you're in luck, the next Monday Night Football game listed on the board will be a matchup pitting a team from the West Coast against a team from the East Coast. For most gamblers, deciding which team to bet on comes down to factors such as hometown loyalty, the trend of the last few games, or which team is on the road. But there's a much easier way to beat the odds: just put money on the team from the West Coast.
Betting tips wouldn't seem to fit in a book about sleep. Yet sleep is the most obvious part of a cycle that affects almost all living things, from quarterbacks in the National Football League to bacteria in the locker-room shower. Living organisms have an innate sense of the length of a day, which isn't all that surprising considering that, for the long history of life on earth, the sun has pretty much run the show. Plants rely on sunlight for energy, while animals have settled into niches based on the span of hours they considered the optimal time to be awake. As a result, somewhere inside the cells of most living things is what amounts to a fairly accurate 24-hour clock, known as the circadian rhythm, which tells an organism when it is time to perform an important activity and when it is time to rest.

The first person to recognize this rhythm was an 18th-century French astronomer named Jean-Jacques d'Ortous de Mairan. Staring at his garden in 1729, he was struck by the fact that his plants extended their leaves in the daytime and then pulled them back at night. He assumed that it was based on sunlight, and he set up a simple experiment to test his theory. He took a bunch of plants down to his wine cellar, where there was no variation of light or temperature between day or night, and recorded the movements of their leaves. Sure enough, the plants continued to open in the morning and close in the evening, even though there was no daylight prodding them. De Mairan realized that the plants were anticipating sunlight rather than responding to it. They had an ingrained sense of when the day started, and they didn't need light to clue them in.

The human body is more like de Mairan's plants than you might expect. The circadian rhythm alters our body temperature and overall alertness level based on the time of day, a process that will make the body continue to stick to the sun's schedule even if it is placed in the equivalent of a wine cellar overnight. Without any help from coffee, most of us tend to perk up around nine o'clock in the morning and stay that way until around two in the afternoon, which is when we start thinking about a nap. Around six in the evening, the body gets another shot of energy that keeps us going until about ten at night. After that, our body temperature starts to fall rapidly, and we get sleepy if we don't turn to coffee or another form of caffeine. Evolutionary biologists don't know exactly why the body operates on this sort of split rhythm, but the best guess is that the early-evening pick-me-up was advantageous to early humans who needed energy to make a fire or find their way back home after a long day of foraging for food.

All of which brings us back to betting on football games. In the middle of the 1990s, a few sleep researchers at Stanford University decided to test a theory. Studies had shown that strength, flexibility, and reaction times surge in the early evening when the circadian rhythm is pulling the body out of the post-lunch funk. Given that subtle effect on an athlete's abilities, it stood to reason that a person at the peak of this alertness cycle would have an unseen advantage over someone whose internal clock thought it was time for bed. What the researchers needed to test their idea was a contest that not only pitted people of similar abilities at different stages of the circadian cycle against each other, but also a data set long enough to show reliable patterns.

They found it in the professional football games played on Monday night, some of the premier events in the NFL. Monday night games always start at 8:30 p.m. Eastern Standard Time, regardless of which teams are playing or which team has to travel. For the league, this guarantees the maximum number of viewers. Diehard football fans on the East Coast will stay up until past midnight if they have to, while sports fans on the West Coast can turn on the TV and watch the game while eating dinner after work.

The scheduling of Monday Night Football games presents a unique circadian problem, especially if a team from the West Coast is playing a team from the East Coast. Players on the West Coast team are playing at their equivalent of 5:30 p.m., no matter if the game is in Seattle or Miami. Players on the team from the East Coast, meanwhile, are three hours ahead in their own circadian cycles. In nature, this sort of mismatch couldn't happen. It was only in the last 60 years or so that we've developed a way to travel so quickly across time zones that our internal clocks are no longer in sync with the daylight around us. Fitting its cause, we call this condition jet lag.

Without knowing it, athletes on teams from the East Coast are playing at a disadvantage. Because of the circadian rhythm, which they can't control, their bodies are past their natural performance peaks before the first quarter ends. By the fourth quarter, the team from the East Coast will be competing close to its equivalent of midnight. Their bodies will be subtly preparing for sleep by taking steps such as lowering the body temperature, slowing the reaction time, and increasing the amount of melatonin in their bloodstream. Athletes on the team from the West Coast, meanwhile, are still competing in the prime time of their circadian cycle.

Every human body, ranging from a professional athlete to a suburban dad, will experience small declines in physical ability and mental agility the longer it fights against the circadian rhythm. In the modern NFL, this has a significant impact because teams in the league are more evenly matched than those in the other major sports, and anything that alters a single player's ability has an outsized effect on the outcome of the game. What's more, there is little that an East Coast team can do about the circadian disadvantage. The schedule gives coaches few chances to adapt to the time difference. Teams traveling on the road typically fly in the night before the game, and East Coast teams playing at home rarely attempt to put their body clocks on Pacific Standard Time. Coaches instead tell their players not to try to adjust to the time differences, preferring that they keep up with their normal sleep patterns for consistency.

The Stanford researchers dug through 25 years of Monday night NFL games and flagged every time a West Coast team played an East Coast team. Then, in an inspired move, they compared the final scores for each game with the point spread developed by bookmakers in Vegas. The results were stunning. The West Coast teams dominated their East Coast opponents no matter where they played. A West Coast team won 63 percent of the time, by an average of two touchdowns. The games were much closer when an East Coast team won, with an average margin of victory of only nine points. By picking the West Coast team every time, someone would have beaten the point spread 70 percent of the time. For gamblers in Las Vegas, the matchup was as good as found money.

In a test to ensure that their findings weren't the result of West Coast teams simply being better during those years, the researchers expanded their scope and looked at every Monday Night Football game played during that twenty-five-year time span. They found that the overall winning percentages for West Coast and East Coast teams were essentially even when the teams were not playing a game against an opponent from the other coast. Nor were the results a reflection of home-field advantage. When an East Coast team traveled to another destination within its same time zone, it won 45 percent of the time. But if a team from the East Coast played somewhere in the Pacific time zone, its winning percentage shrunk to only 29 percent.

The circadian advantage—or disadvantage, depending on your perspective–-popped up in studies of figure skaters, rowers, golfers, baseball players, swimmers, and divers. Everywhere you turned, there was evidence of the body's hidden rhythms at work. One study found that in sports as varied as running, weightlifting, and swimming, athletes competing when their bodies experienced the second boost of circadian energy were more likely to break a world record. Long jumpers, for instance, launched themselves nearly 4 percent farther when the body was at its circadian peak. But the circadian rhythm cut both ways. Athletes competing when their circadian rhythm corresponded to the so-called sleep gates—those times in the early afternoon or late nights when it's easy for most people to fall asleep—consistently performed a little worse than normal, even if the slowdown wasn't obvious to them.